Turboprop engine idling control



April 4,

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TURBOPROP ENGINE IDLING CONTROL Filed Aug. 16, 1955 2 Sheets-Sheet 2 3& W

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ATTORNEY United States Patent 6 Robert J. Wente, Indianapolis, Ind,, assignor to General Motors Corporation, Detroit, Mich., a corporation of a Delaware I Filed Aug. 16, 1955, Ser. No. 528,703 3 Claims. (c1. 170 -13502 This-invention is directed to improved controls'for gasturbine propeller aircraft engines, commonly called turboprop engines. It is particularly directed to improving the landing characteristics of airplanes propelled by such engines. 1

In general, the problem of controlling a turboprop engine is a rather complicated and difficult one. In flight, it is advantageous to control engine speed by a variable pitch speed governing propeller and control the power developed, and hence the propulsive effort, by varying the flow of fuel to the engine. Such a control requires safeguards, 'however.- It must, if it is to function acceptably, be such that it prevents overtemperature conditions in the engine, prevents cutting fuel flow below the minimum at which combustion will be maintained, and schedules acceleration or deceleration'of the' engine so as to prevent surge, stall, transient excess temperature conditions,'or flameout; of the engine. A suitable control also must compensate forchanges in temperature a danger point, and, in most cases, an overspeed governor. t

There are known controls for gas turbines, operating as indicated above, which meter fuel flow to theengine in response to three control parameters; 'engine speed,

total pressure of inlet air, and total temperature of inlet 'Such controls, however, so far as I am aware, have heretofore converted these parameters into fuel flow so asto maintain turbine inlet temperature, constantuat any given control setting. i t L Of course, it is possible to controlan engine on the basisof other parameters, but this is the preferred system. It is satisfactory for-flight at full power or'under cruising conditions, but ithas. been found to be unsatisfactory during landing approaches, which should be. made with the engine idling. In one installation 'it hasgbeen found desirable to make the landing approach with the engine developing slightly less; than zero output; specifically, taking, from. the propeller about five ,per cent of the maximum engine power rating. 1"

-, For best performance in landing, the power low speed flight or landing approach characteristics of the particular aircraft. In this discussion, it ,should be understood that the term power'output may refer to a, negative power output as well'asgapositiveone, and that; idlingfloperation is operation with poweroutput near" .zero. 1 f Q Principally because of the difierent m ent tempera.

I 7 output of. the engine should be maintained constant during the? landing approach. at a'value whichis consonant with the r 2,918,024 Patented Apr. 4, 1961 turesrencountered during landing approaches, and the operating characteristics of gas turbines, the prior art fuel control metering fuel to the engine so as to maintain turbine inlet temperature substantially constant will not maintain constant idling horsepower.

The control system of this invention will provide substantially constant idling power output of the engine dur ing a landing approach on the basis of the three controlling parameters of inlet pressure and temperature and engine speed. By virtue of this improved mode of operation, the aircraft may be brought in on a landing approach at a constant power lever setting and it is not necessary for the pilot to devote constant attention to adjusting the power output of the engine to keep the aircraft in the rather narrow range of speeds slightly above stalling speed suitable for landing.

The principal objects of the invention are to improve the operating characteristics of gas turbine powered aircraft and to improve controlling methods and means for turboprop engines.

The manner in which theseobjects are achieved and the advantages of the invention will be apparent to those skilled in the art from the succeeding detailed description of the preferred mode of carrying out the invention and the preferred embodiment of means embodying the invention, illustrated by the accompanying drawings, in which:

Figure l is a chart illustrating the fuel flow and turbine temperature characteristics of a typical turboprop engine as a function of ambient temperature at constant idling horsepower;

Figure 2 is a graphical illustration of the variation with: power lever position and ambient temperature of engineshaft horsepower and turbine inlet temperature of a' turboprop engine controlled in accordance with the invention;

Figure 3 is a diagram illustrating variation in fuel flow with power lever position and ambient temperature;

Figure 4 is an elementary schematic diagram of a gas turbine fuel system; and

Figure 5 is a schematic diagram of the relevant parts of a fuel control embodying the invention.

Referring first to Figure 1, this is a diagram illustrating the variation of turbine inlet temperature and fuel' flow to maintain constant idling power output of an engine as ambient temperature varies from 60 to 120 F.,, engine r.p.m. being constant. There are two curves for" each of these variables, one representative of variation? at zero air speed and the other indicating variation at an; air speed of three hundred knots. As will be apparent, as ambient temperaturev increases, turbine inlet tempera-- ture for constant idling power output increases quite substantially.-* Contrariwise, to maintain constant power output, fuel flow must decrease as the ambient temperature increases. The fuel required and turbine inlet temperature will both be lower at the three hundred knot air speed because of the contribution of ram effect of air entering the compressor to engine power output. Looked at another way, this means that when the aircraft slows down the fuel requirement increases slightly; if this'requirement is not met, power will decrease. over the landing approach speed range, this effect is inconsiderable. I

I power of theengine as ambient temperature increases.

Thiswould be accomplished, of course, .by..decreasing fuel flow ,as turbine inlet temperature tends to increase with higher ambient, temperature 'so: as to maintain the temperature constant. In;other words, the fuel now However,

3 curve would have a steeper slope than that in Figure 1. This condition is also very likely to be encountered in landing where air temperature increases rapidly as the ground is approached.

Of course, at higher power settings of the engine, it is not practicable to maintain constant shaft horsepower because of the resulting wide variations in turbine inlet temperature, the primary limiting factor of engine oper- ,ation at such normal power output levels.

Referring now to Figure 2, there is illustrated a schedule of turbine inlet temperature and shaft horsepower as a function of power lever position. The power lever is the lever moved by the pilot or flight engineer to determine engine output and, as is customary, its movement is referred to in terms of angles from zero to 90. In the example illustrated, the maximum power output is at 90. Cruising positions are in the range from 90 down to about 45, and idling position is at 30. Below 30 may be a ground maneuvering range of operation of the power plant at fixed propeller blade angle which is not affected by the present invention and, therefore, does not require analysis. It will be understood that above the 45 power lever position the power control sets a turbine inlet temperature. The fuel control acts to meter fuel to maintain this turbine inlet temperature and the power output of the engine is whatever the characteristics of the engine determine. It will be noted from Figure 2 that above the 45 position of the power lever the shaft horsepower of the engine increases with advance in the power lever position and that the .power output is very strongly affected by ambient temperature. The characteristic of the engine is such that for constant turbine inlet temperature the power increases with decreasing ambient temperature. Thus, for example, the power output at the 45 position at 60 ambient temperature is about 60 percent of the maximum power output at 120 ambient temperature. With 120 ambient temperature and 45 power lever position, power output is substantially zero. The temperature curve illustrates the turbine inlet temperature corresponding to all of the four curves of shaft horsepower above the 45 control position. It will be noted that the increase in power is a function of a gradually increasing turbine inlet temperature. If this turbine inlet temperature curve were extrapolated below the 45 control position, it would correspond to the temperature line marked 60 corre sponding to 60 F. ambient temperature. In this case, the various shaft horsepower curves would remain widely separated below the 45 control position.

In the control according to the invention, the mode F of control gradually changes between the 45 control position and the 30 or idling control position so thatv the engine, since the idling is in alow and safe tempera ture range.

Figure 3, which is a graph of typical fuel metering conditions as a function of, power leverpositions and ambient temperature, shows a fuel flow control schedule to obtain the result illustrated by Figure 2.

Fuel is controlled to operate the engine at constant horsepower at the idling condition while operating it at constant turbine inlet temperature through the normal flight, range. As will be apparent, the curve for 60 temperature, which is standard, is a gradual consistent curve through the entire range from maximum power to idle.

However, the fuel" flow: curves for the. lower and thus decrease power at idling, whereas the curve for 120 ambient temperature slopes less steeply and rises toward the standard or 60 curve as the 30 power lever position is approached.

It will be apparent from the foregoing how the control method of the invention effects a transition from control to maintain desired turbine inlet temperature under normal flight conditions to a control to maintain substantially constant power output during idling conditions such as are suitable forlanding approaches. As a result, the performance of the aircraft in landing is consistent and safe and does not reguire undue attention from flight or ground personnel.

An embodiment of the invention in fuel control structures is illustrated in Figures 4 and 5. Referring first to Figure 4, there is illustrated an engine E coupled to a propeller W of the speed governing variable pitch type through a shaft 10. Shaft 10 is geared to an accessory drive shaft 11 which in turn is geared to a speed input shaft 12 of a fuel control 14 and to drive shaft 16 of a fuel pump 17. The fuel control also has an input of total presure in the engine air inlet, indicated by the line P, and an input of temperature in the air inlet, indicated by the line T. Thereis also a power control input to the fuel control indicated by the lever C coupled by linkage 18 to the pilots engine control lever 20.

Fuel from a suitable source is supplied through line 21 t0 the pump, which discharges it through line 22 to the fuel control 14, which meters the fuel and delivers the required quantity to the engine through line 23. The fuel control includes a throttle or metering valve through which the fuel passes .to the engine and a by-pass valve which maintains constant pressure across the throttling valve. For reasons of clarity, the by-pass valve 24 is shown as a separate element in Figure 4. This by-pass valve communicates with the upstream and downstream sides of the throttle valve through lines 26 and 27, respectively, and acts to maintain constant pressure across the throttle valve by returning the necessary portion of the pumped fuel to the pump inlet through aline 28. Such fuel metering systems are generally known and'the relation of a by-pass valve to fuel metering valves is illustrated, for example, in British Patent 727,201.

The mode in which the invention is carried into effect will be more clearly apparent from further consideration of the preferred nature of the fuel control 14, the significant parts of which are illustrated schematically in Figure 5.

Referring to Figure 5, the control input lever C of the fuel control operates a throttle setting lever 30 through suitable means such as a cam (not shown). The speed input shaft 12 operates two speed responsive devices of the fly-ball type (not shown) one of which exerts a governor weight force against a plate 31 on a valve operating shaft 32. The governor weight force is resisted by spring 33 variably loaded by a governor setting lever 34 suitably coupled to input lever C, as by cam mechanism (not illustrated). The second speed response device exerts a speed weight force indicative of engine speed against a disk 35 on a reciprocable shaft 36 biased by a spring 37. The governor which moves shaft 32 is a governor in the usual sense of the word in that it may act to limit or control engine shaft speed under taxiing ambientternperatures break downwardly to reduce'flow' or fixed propeller pitch-conditions. It'also may act as an emergency limiting, governor in event of failure of failure of the propeller governor. The speed weight force input to the shaft 36 is a control input for the fuel metering.

' Fuel metering is also responsive, as previousiy stated; to temperature 'in the-engine inlet through suitable mechanism'such as 'a bulb in the' air inlet (not shown) con nected through a capillary tube 41 to a' bellows 42. Bellows 42, when it expands, urges a rotatable and' axially shiftable camshaft 43. to the left in the figure against the a'ction of-a spring-4,4 acti'ng througha; roclcer arm 46 of the:orifice.j-fl

The axial shifting of the valve is moreicomplexts :How-

, fixed on the camshaft.

Therefore, camshaft 43 is turned in response to engine speed and is shifted axially in response to inlet temperature. Camshaft 43 bears a part throttle cam 50 and an acceleration limit cam 52. The acceleration limit cam is provided tov control transient conditions of the engine during changes in power control settings. It may be .engaged by a follower 53 extending from a sleeve 54 rotatably mounted on a shaft 56. The sleeve 54 is integral with an arm 57 which has a lostmotion connection with an arm 58 on a second sleeve 59 rotatably mounted on' shaft 56. A cam. follower 61 is integral with a slidable collar 62 splined to the sleeve 59. Collar 62 is shifted axially of thesleeve 59 by a pin 63 extending from the throttle setting lever'30. As will be apparent,.axial movement of camshaft 43 by the tem-" peratureresponse and, likewise,-shifting the follower 61 through operation of the control input C, changes the zone of cam 50 which is engaged by follower 61. v

Cam 50 is normally coupled through follower 61, coller. 62, sleeve 59 and engaging arms 58 and 57 to sleeve 54, which has an arm 66 which normally bears against a disk 67. on valve shaft 32. A rotatable and axially shiftable metering valve member 70 mounted on shaft- 32, is formed with a generally rectangular opening 71 which cooperates with a rectangular opening 72 in an outer valve sleeve 73 to define a variable orifice which is the fuel metering orifice or throttle valve of the fuel control. Fuel flows radially from within the valve sleeve through the orifice defined by the openings 71 and 72, which is in the path between lines 22 and 23 of Figure ,4. The area of the orifice may be varied by either axial or rotary movement of sleeve 70. Axial movement is controlled by the speed governor weight force, the force, of spring 33, and the action of the cams onshaft; 43. Rotary movement is responsive to the pressure input P of Figure (4 which preferably acts through a suitable servomechanism, the inlet pressure compensating servo 78, which is controlled in any known manner so that it acts to move the piston rod 79 of the servo in response to total pressure in the airinlet. The piston .rod 79 has rack teeth on it to cooperate with a gear sector 81=ori the sleeve 70 sothat it is rotated to open thevalve as inlet pressure-increases. The action, ofinlet pressure is relatively simpleinthat it acts alone to vary one. dimension ever, normally this valve is controlled by cam 54 acting through, cam. follower; 61 as previously described against thetresistauce of spring-33: r-Under-these conditions, the, acceleration limitzcam islnot effective. However, under transient conditionsofoperation of'the engine, the ac celeration;limit cam may engage follower 53 to rotate" sleeve 54:c'lockwise: as illustrated in Figure "5, separating abutting arms 57and 58 and overriding the actioniof cam 50 in adirection to close the-valve and thus liniit' fuel; flow.- If, forany "reasonjsuc-h as failure of the propeller governor} engine speedin'creases to the poi'nt where the governor weight force will overcome theset ting of spring'33y the'governor actsto move shaft 32an d valve sleeve:70"tottheright to decrease fuel flow, the

disk 67 'rnoving away fromar'm 66 in; thisea'ep ln flight,thespee'dgovernqr is setihiglier tharr thepropeller i governor" andis operative ionly inf the 7 event of failure of theprOpelIeigovernor. However,'thi's ;gov:

1 ernor also provides for operation ofthe power plant fixedblade angle [operationjfor taxiing and braking, in

' Whichcasethe' g ernor 'nor'r'nally may limit engine speed propeller governor loses control.

7 5; gine speed below theno'rmal flight 1pm." acts through 6 and thus be in command of the throttle. This mode of operation of the engine is not material to the present in",- vention, which is usable under different conditions and whether or not any fixed blade angle operation of the engine is available.

Considering the operation "of the system and assuming that the airplane is in normal flight, the pilot will have his engine control lever 20 in a flight or cruise setting calling for normal power output of the engine. This setting of control lever 20 operates through the input arm C of the fuel control, and fuel control will be at a high power setting with cam follower 61 toward the left or near end of calm 50 as shown in Figure 5. If the pilot wishes to operate the plane in flight at low speed, he moves the control 20 to call for lower power and cam follower 61 will be moved toward the intermediate zone of the cam for lower power settings. The combined effect of the inlet pressure, inlet temperature, and speed senses acts to supply fuel so as to maintain constant turbine inlet temperature. Engine speed normally does not vary because it is determined by the propeller governor and, in the power plant described herein, constant speed operation of the propeller throughout the available range of power for flight purposes is the preferred mode of operation.

It will be seen, therefore, that in normal flight the pilot acts to maintain a variable engine power level conditioned primarily on a scheduled turbine temperature. Such control could,of course, be exercised by response by the pilot to an indication of turbine temperature and corresponding movement of the control 2t) to hold the temperature at the desired value. Such a mode of op eration, depending on direct response by the pilot to turb-ine temperature, is impractical, particularly for multiengine aircraft. Therefore, all practical installations embody power control means which may be such as that describedfherein in which the turbine temperature is maintained more or less accurately at a value determined by the pilot by setting the control lever 20. r

Assuming now that the plane approaches for a landing, the throttle will be cutback by the pilot, the power control lever being moved to the 30 position which modified so as to maintain constant power instead of constant turbine inlet temperature. With the airplane in the landing glide the propeller remains in governing operation and windmills so as to provide some power to the engine. Because the power output of the engine is kept constant, the windmilling drag of the propeller remains constant at the various ambient conditions. This constant enginepower output could bemaintained with any elementary fuel throttling means by providing the pilot with. an indication of engine power output and having the pilot adjust the control lever 20 for each engine dur-' ing the landing approach to hold the power at .the desired level. 'This would be impractical, since it would call for too much attention to engine control at a time when there are many other things tobe attended to by the pilot. Ordinarily, at about touchdown, the speed of the aircraft decreases to a point at which the governor brings the propeller against a low pitch stop so thatthe As the speed of the aircraft decreases on the runway the power input' from the propeller to the engine decreases and, therefore, the engine speed tends to decrease.

In this phase of operation, however, the part throttle cam SQ'is socontouredthat substantial. decrease of enthe speed weight servo 47, rotating camshaft 43, to rotate cam .50 under follower 61. The cam. is socon- "toured that this rotation. increases fuel How to the engine with decreasing engine speed. In this regime the speed weight input and the mechanism actuated thereby thus serves as an underspeed governor to keep engine power output up to the demand imposed by the propeller. This underspeed governor mode of operation also may be used in taxiing and braking operation of the engine at fixed blade angle, since increase in load on the engine by increasing pitch of the propeller tends to slow the engine, which causes the underspeed goverenor to increase fuel and minimize the drop in speed consequent upon the imposition of the higher load.

It may be noted that the response of the fuel control to engine inlet pressure remains the same in all modes of operation. Increase in ambient pressure tends to increase the power output of the engine, but since such increase in ambient pressure is accompanied by an increase in the drag of the aircraft these two effects are substantially mutually compensatory.

As stated, the value of the constant power operation is primarily in the improved control and safety of the aircraft during the landing approach. This valuable feature has been added to the previously known turbine control systems without injuring the performance of the engine or hazarding its components in the higher powered ranges used for normal flight.

As will be apparent to those skilled in the art, the principles of the invention may be embodied in various fuel control systems and the scope of the invention is not to be considered as limited by the detailed description herein of the preferred embodiment thereof.

I claim:

1. The method of operating a gas turbine propeller aircraft propulsion power plant comprising regulating the fuel supply to the power plant to maintain turbine temperature substantially constant notwithstanding variations in ambient atmospheric temperature, and varying propeller pitch to absorb the engine power output at substantially constant speed, over a flight range from maximum power to a low power condition; and regulating the fuel supply to the power plant to maintain engine power output substantially constant notwithstanding variations in ambient atmospheric temperature, at a low power setting suitable for aircraft landing approaches.

2. The method of operating a gas turbine propeller aircraft propulsion power plant comprising regulating the fuel supply to the powere plant to maintain turbine temperature substantially constant notwithstanding variations in ambient atmospheric temperature, and varying propeller pitch to absorb the engine power output atsubstantially constant speed, over a flight range from maximum power to a low power condition; regulating the fuel supply to the power plant to maintain engine power outputsubstantially constant notwithstanding variations in ambient atmospheric temperature, at a low power setting suitable for aircraft landing approaches; and limiting the minimum value of propeller pitch at the lower power setting. I

3. The method of operating a gas turbine propeller aircraft propulsion power plant comprising regulating the fuel supply to the power plant to maintain turbine temperature substantially constant notwithstanding. variations in ambient atmospheric temperature, and varying.

propeller pitch to absorb the engine power output at substantially constant speed, over a flight range from maximum power to a low power condition; regulating the.

low power seting in accordance with engine speed when engine speed approaches a minimum desired value.

4. The method of operating a gas turbine propeller aircraft propulsion power plant comprising regulating the fuel supply to the power plant to maintain turbine temperature substantially constant notwithstanding variations in ambient atmospheric temperature, and varying propeller pitchto absorb the engine power output at substantially constant speed, over a flight range from maximum power to a low power condition; and regulating the fuel supply to the power plant to maintain engine power output substantially constant notwithstanding variations in ambient atmospheric temperature, while varying propeller pitch subject to' a fixed low pitch limit, at a low power setting suitable for aircraft landing approaches. 5. A fuel control for a gas turbine aircraft engine comprising, in combination: fuel metering means; means responsive to a condition representative of ambient atmospheric temperature; settable power control means for controlling the power output of the engine; means coupling the responsive means to the metering means, the coupling means being connected to and controlled by the power control means, the coupling means including means operative at the idling power setting of the power control means to vary fuel flow as a function of ambient atmospheric temperature so as to maintain engine power output substantially constant notwithstanding variations in ambient atmospheric temperature and means operative at power settings above the idling power setting of the power control means to vary fuel flow as a function of ambient atmospheric temperature so as to maintain turbine temperature of the engine substantially constant notwithstanding variations in ambient atmospheric temperature.

6. A fuel control for a gas turbine aircraft engine coupled to a speed-governing propeller, the control comprising, in combination: fuel metering means; means responsive to a condition representative of ambient atmospheric temperature; settable power control means for controlling the power output of the engine; means coupling the responsive means to the metering means, the coupling means being connected to and controlled by the power control means, the coupling means including means operative at the idling power setting of the power control means to vary fuel flow as a function of ambient atmospheric temperature so as to maintain engine power output substantially constant notwithstanding variations in ambient atmospheric temperature and means operative at power settings above the idling power setting of the power control means to vary fuel flow as a function of ambient atmospheric temperature so as to maintain turbine temperature of the engine substantially constant notwithstanding variations in ambient atmospheric temperature.

7. A fuel control for a gas turbine aircraft engine coupled to a speed-governing propeller, the control comprising, in combination: fuel metering means; means responsive to a condition representative of ambient atmospheric temperature; settable power control means for controlling the power output of the engine; means coupling the responsive means to the metering means, the coupling means being connected to and controlled by the power control means, the coupling means including a first portion operative at the idling power setting of the power control means constructed to vary fuel flow as a function of ambient atmospheric temperature so as to maintain engine power output'substantially constant notwithstanding variations in ambient atmospheric temperature 7 and a second portion operative at power settings above in. ambient atmospheric temperature, at a low power seting suitable for aircraft landing approaches; limiting the minimum value of propeller pitch at the low power set ting; and regulating the fuel, supply to the engine at the the idling power setting of the power control means constructed to vary fuel flow as a function of ambient atmospheric temperature so as to maintain turbine temperature of the engine substantially constant notwithl standing. variations in ambient atmospheric temperature.

8. 'A fuel control for a gas turbine aircraft engine coupled to a speed-governing propeller, the control comprising, in combination: fuelmetering means; means responsive to a condition representative of ambient atmospheric temperature; settable power control means for controlling the power output of the engine; means coupling the responsive means to the metering means, the coupling means being connected to and controlled by the power control means, the coupling means including a first cam portion operative at the idling power setting of the power control means contoured to vary fuel flow as a function of ambient atmospheric temperature so as to maintain engine power output substantially constant notwithstanding variations in ambient atmospheric temperature and a second cam portion operative at power settings above 10 the idling power setting of the power control means contoured to vary fuel flow as a function of ambient atmospheric temperature so as to maintain turbine temperatuire of the engine substantially constant notwithstanding variations in ambient atmospheric temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,628,472 Dray et a1. Feb. 17, 1953 2,638,742 Carey May 19, 1953 2,640,550 Knapp et a1, June 2, 1953 2,667,228 Wood et al. Jan. 28, 1954 2,759,549 Best Aug. 21, 1956 

